FILEASSOC(9) NetBSD Kernel Developer's Manual FILEASSOC(9)
NAMEfileassoc -- in-kernel, file-system independent, file-meta data associa-
tion
SYNOPSIS#include <sys/fileassoc.h>DESCRIPTION
The fileassoc KPI allows association of meta-data with files independent
of file-system support for such elaborate meta-data.
When plugging a new fileassoc to the system, a developer can specify pri-
vate data to be associated with every file, as well as (potentially dif-
ferent) private data to be associated with every file-system mount.
For example, a developer might choose to associate a custom ACL with
every file, and a count of total files with ACLs with the mount.
Kernel Programming Interface
Designed with simplicity in mind, the fileassoc KPI usually accepts four
different types of parameters to the most commonly used routines:
struct mount * mp
Describing a mount on which to take action.
struct vnode * vp
Describing a file on which to take action.
fileassoc_t id
Describing an id, as returned from a successful call to
fileassoc_register().
void * data
Describing a custom private data block, attached to either a file
or a mount.
Before using the fileassoc KPI it is important to keep in mind that the
interface provides memory management only for fileassoc internal memory.
Any additional memory stored in the tables (such as private data-struc-
tures used by custom fileassocs) should be allocated and freed by the
developer.
fileassoc provides the ability to specify a ``cleanup'' routine to
fileassoc_register() (see below) to be called whenever an entry for a
file or a mount is deleted.
Fileassoc Registration and Deregistration Routines
These routines allow a developer to allocate a fileassoc slot to be used
for private data.
intfileassoc_register(const char *name, fileassoc_cleanup_cb_tcleanup_cb, fileassoc_t *result)
Registers a new fileassoc as name, and returns a fileassoc_t via
result to be used as identifier in subsequent calls to the
fileassoc subsystem.
fileassoc_register() returns zero on success. Otherwise, an
error number will be returned.
If cleanup_cb is not NULL, it will be called during delete/clear
operations (see routines below) with indication whether the
passed data is file- or mount-specific.
cleanup_cb should be a function receiving a void * and returning
void. See the EXAMPLES section for illustration.
intfileassoc_deregister(fileassoc_t id)
Deregisters a fileassoc whose id is id.
Note that calling fileassoc_deregister() only frees the associ-
ated slot in the fileassoc subsystem. It is up to the developer
to take care of garbage collection.
Lookup Routines
These routines allow lookup of fileassoc mounts, files, and private data
attached to them.
void *fileassoc_lookup(struct vnode *vp, fileassoc_t id)
Returns the private data for the file/id combination or NULL if
not found.
Mount-wide Routinesintfileassoc_table_delete(struct mount *mp)
Deletes a fileassoc table for mp.
intfileassoc_table_clear(struct mount *mp, fileassoc_t id)
Clear all table entries for fileassoc from mp.
If specified, the fileassoc's ``cleanup routine'' will be called
with a pointer to the private data-structure.
intfileassoc_table_run(struct mount *mp, fileassoc_t id, fileassoc_cb_tcb, void *cookie)
For each entry for id, call cb with the entry being the first
argument, and cookie being the second argument.
cb is a function returning void and receiving one void * parame-
ter.
File-specific Routinesintfileassoc_file_delete(struct vnode *vp)
Delete the fileassoc entries for vp.
If specified, the ``cleanup routines'' of all fileassoc types
added will be called with a pointer to the corresponding private
data structure and indication of FILEASSOC_CLEANUP_FILE.
Fileassoc-specific Routinesintfileassoc_add(struct vnode *vp, fileassoc_t id, void *data)
Add private data in data for vp, for the fileassoc specified by
id.
If a table for the mount-point vp is on doesn't exist, one will
be created automatically. fileassoc manages internally the opti-
mal table sizes as tables are modified.
intfileassoc_clear(struct vnode *vp, fileassoc_t id)
Clear the private data for vp, for the fileassoc specified by id.
If specified, the fileassoc's ``cleanup routine'' will be called
with a pointer to the private data-structure and indication of
FILEASSOC_CLEANUP_FILE.
EXAMPLES
The following code examples should give you a clue on using fileassoc for
your purposes.
First, we'll begin with registering a new id. We need to do that to save
a slot for private data storage with each mount and/or file:
fileassoc_t myhook_id;
int error;
error = fileassoc_register("my_hook", myhook_cleanup, &myhook_id);
if (error != 0)
...handle error...
In the above example we pass a myhook_cleanup() routine. It could look
something like this:
void
myhook_cleanup(void *data)
{
printf("Myhook: Removing entry for file.\n");
...handle file entry removal...
free(data, M_TEMP);
}
Another useful thing would be to add our private data to a file. For
example, let's assume we keep a custom ACL with each file:
int
myhook_acl_add(struct vnode *vp, struct myhook_acl *acl)
{
int error;
error = fileassoc_add(vp, myhook_id, acl);
if (error) {
printf("Myhook: Could not add ACL.\n");
...handle error...
}
printf("Myhook: Added ACL.\n");
return (0);
}
Adding an entry will override any entry that previously exists.
Whatever your plug is, eventually you'll want to access the private data
you store with each file. To do that you can use the following:
int
myhook_acl_access(struct vnode *vp, int access_flags)
{
struct myhook_acl *acl;
acl = fileassoc_lookup(vp, myhook_id);
if (acl == NULL)
return (0);
error = myhook_acl_eval(acl, access_flags);
if (error) {
printf("Myhook: Denying access based on ACL decision.\n");
return (error);
}
return (0);
}
And, in some cases, it may be desired to remove private data associated
with an file:
int error;
error = fileassoc_clear(vp, myhook_id);
if (error) {
printf("Myhook: Error occurred during fileassoc removal.\n");
...handle error...
}
As mentioned previously, the call to fileassoc_clear() will result in a
call to the ``cleanup routine'' specified in the initial call to
fileassoc_register().
The above should be enough to get you started.
For example usage of fileassoc, see the Veriexec code.
CODE REFERENCESsrc/sys/kern/kern_fileassoc.cHISTORY
The fileassoc KPI first appeared in NetBSD 4.0.
AUTHORS
Elad Efrat <elad@NetBSD.org>
Brett Lymn <blymn@NetBSD.org>
NetBSD 5.0_RC4 May 15, 2007 NetBSD 5.0_RC4

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